A known hydrodynamic torque converter disclosed in US2006/0185954A1 (hereinafter referred to as Reference 1) includes an impeller clutch. A known lock-up device for a hydraulic torque converter including an impeller clutch is disclosed in JP2007-113659A (hereinafter referred to as Reference 2). According to References 1 and 2, a driving force of an engine may be transmitted and disconnected by the impeller clutch to and from a pump impeller of the torque converter through an independent oil passage. Transmission and disconnection of the driving force between the engine and the pump impeller is controlled by a clutch engagement pressure supplied to the impeller clutch through the independent oil passage. In a case where a vehicle is expected to start moving, the clutch engagement pressure is applied to the impeller clutch via the independent oil passage. Then, the impeller clutch is brought into an engaged state to therefore bring the vehicle into a state to start moving.
The aforementioned independent oil passage is provided at a different position from an oil passage via which a pressure is applied to the torque converter disclosed in References 1 and 2. Accordingly, the impeller clutch having the independent oil passage may be easily controlled. The pressure applied to the torque converter corresponds to an internal pressure of the torque converter.
However, in the impeller clutch according to References 1 and 2, the internal pressure of the torque converter acts as a back pressure relative to the impeller clutch and thereby causes the impeller clutch to require time to reach a completely engaged state after the clutch engagement pressure is applied to the impeller clutch. As a result, a time lag occurs between the time when the impeller clutch is brought into the completely engaged state and the time when the vehicle is brought into the state to start moving.
A need thus exists for a control mechanism and a method for engagement of an impeller clutch, which are not susceptible to the drawback mentioned above.